Strange click when backing up

[romac]

I have a 2006 V6 Mustang, manual trans. After the car has been sitting over night or for a few hours, when I start it and put it in reverse and start backing up there's a clicking noise under the car, almost like something is catching or something. It only does this once, and only after the car has sat for a while. Any idea what this is? Anything to worry about at all? It runs and drives fine.